Static electricity is estimated to cost industry billions of dollars annually from damaged semiconductors and printed circuit boards. A support industry has developed that provides conductive, static shielding, and antistatic materials for electronics packaging and materials handling. Static control plastics include, for example, fiber, chemical or particle filled thermoplastics which are fabricated into films, bags, tote boxes, module carriers, dip tubes, and the like.
However, requirements for static control can vary. Generally, resistivities less than 1012 ohms (surface) are suitable and below 1010 ohms are ideal. A further advantage would be the incorporation of a Faraday Shield (a conductive layer or grid that reduces exposure of packaged electronic components to electrostatic "fields" as opposed to "discharges").
Heretofore, the art in static control has, primarily, consisted of two separate and distinct technologies, which can not be advantageously combined. The first conventional technology involves surface active systems including amine blooming agents numectants and surfactants. These systems perform by absorbing ambient moisture at the polymer structure surface forming an aqueous electrolyte microlayer capable of conducting dissipating, and/or attenuation electric charge along the structure's surface. Structures like this have no volume conductive properties.
The second conventional technology in static control involves the incorporation of conductive fillers. It is common to incorporate fillers like fibers, particles, and flakes into thermoplastic resins for reinforcement or modification of the resins' bulk mechanical properties. Similarly, electrically conductive fibers, for example, carbon black, powdered graphite, or graphite fibers, can be incorporated into thermoplastic resins to lend bulk conductive properties to the resin. Whether conductive carbon, carbon fibers, metal flakes, metal-coated fibers, or metal fibers are used, in all cases the additive levels must be high enough to assure particle to particle contact in the resin. In other words, in the case where fillers are used in a resin, the critical factor is volume loading, i.e., it is required that the conductive particles or fibers touch (or very nearly touch) each other to establish electrical continuity through the plastic structure. The resulting conductivity is strictly a function of the inherent conductivity of the conductive additive or filler. For example, when using carbon black as a filler, depending on particle surface area, particle diameter, and on blending parameters, the volume loadings required usually exceed 20 volume %, often ranging as high as 40 volume %. Such high loadings of carbon particles result in compromised physical properties, a black opaque material, and a material that can potentially slough carbon particles. Sloughing is a source of contamination of much concern in the electronics industry.
Conventional graphite or carbon fibers, due to their high dimensional aspect ratio (length to diameter, 1/d), can be added to significantly lower volume concentrations and still give bulk conductive properties to thermoplastics. Again, the magnitude of conductivity achieved depends on loading, fiber shape and size, compounding parameters, and the inherent conductivity of the fibers.
Typically, conventional conductive fiber loadings as low as 5 to 10 volume % have been used. The critical factor is fiber length, i.e., the longer the fibers the more likely they will touch, completing the electrical circuit. However, in the past the retention of fiber length during processing of the composition has been a key problem. Shear forces in processing equipment break the fibers, shortening their effective 1/d and reducing their usefulness. To compensate, a higher volume of fiber is added to the resin, contributing to material cost increases, polymer structure stiffening, opacity, and process complexity.
Further complicating matters, conventional fibers become more conductive through heat treatment, and with increasing inherent fiber conductivity comes both reduced fiber flexibility and increased fiber brittleness. Therefore, optimally one would like to compound into a thermoplastic a highly conductive fiber at low volume levels. However, the highly conductive fibers are more easily broken during processing. Hence, either the inherent conductivity or the volume added must be sacrificed to achieve repeatable, useful conductive thermoplastic structures.
U.S. Pat. No. 4,837,076 to McCullough et al, which is herein incorporated by reference, discloses a class of carbonaceous fibers used in the present invention.
U.S. Pat. No. 4,602,051 to Nabeta et al, which is herein incorporated by reference, discloses a resin composition with conventional carbon fibers which has an electromagnetic wave shielding effect. There is further disclosed a kneading and extrusion process which may be utilized in the present invention.
U.S. Pat. No. 4,678,699 to Kritchevsky, et al, which is herein incorporated by reference, relates to stampable thermoplastic composites containing a shielding layer against electromagnetic and radio frequency waves. There is utilized fibrous conductive filler materials which may also be used in the present invention.
The article entitled "EMI Shielding Through Conductive Plastics" by Simon, R. M., Polym. Plast. Technol. Eng., 17(1), 1-10 (1981), which is herewith incorporated by reference, discloses conductive plastics which can be utilized in the present invention.
It is therefore, desired to provide a novel composition that when prepared by conventional processing equipment, eliminates or ameliorates many of the problems associated with the prior art fibers and processes for producing the composition.
It is further desired to provide novel static control materials and structures with the following advantages over the prior art materials and structures.
It is further desired to provide novel static control materials and structures with surfaces having enhanced conductivity (less than 1010 ohms) without the presence of conductive particles or fibers at structure surfaces where they are potential contaminants.
It is further desired to provide novel static control materials and structures in which their surface resistivities can be controlled. By using differing thicknesses of film or extrudate on the outer surfaces, the surface electrical properties can be reproducibly adjusted.
It is further desired to provide novel static control materials and structures with thick rigid thermoplastic sheets which can be readily drawn-down without significant loss of electricals at edges and in corners.
It is further desired to provide novel static control materials and structures with special elongation properties of the fiber comprising the mat.
It is further desired to provide novel static control materials and structures with surface resistivities below 1010 ohms which can be achieved with filament loadings less than 1 volume %. This has very significant impact on the cost of delivered structures, since conductive fiber can make up more than 90% of the material cost in compounded, filled systems.
It is further desired to provide novel static control materials and structures which by concentrating the fiber in one thin layer (still achieving surface enhancement), it is possible to dramatically improve the translucency of structures. Present carbon black loaded structures are always opaque. The present invention reduces the amount of carbon black necessary.
It is further desired to provide novel static control materials and structures with surface electrical properties which are easily achieved and are two orders of magnitude better than prior art systems using surface systems such as amines, etc. as described above.
It is further desired to provide novel static control materials and structures with electrical properties which are permanent and non-humidity dependent. Conventional surface systems are not permanent and humidity dependent.
It is further desired to provide novel static control materials and structures which may be washed with no effect on electrical properties.
It is further desired to provide novel static control materials and structures which are stable with heat (160.degree. F.) in aging experiments (12 days). None of the prior art surface systems has passed this test.
It is further desired to provide novel static control materials and structures which can readily be varied to provide some physical reenforcement to structures by using high tensile mat co-fibers.
It is further desired to provide novel static control materials and structures in the form of a mat to form structure inner layer(s) to allow for a variety of processing plans. The use of layered structures will avoid the use of loose fiber or carbon particles when necessary, will promote a homogeneous dispersion, and retain an optimum fiber length. When fibers are compounded as fillers in the prior art systems, the fiber length is always reduced by shearing and the resulting electricals worsen or at least become less predictable.
It is further desired to provide novel static control materials and structures prepared by calendering of an entangled mat to allow the fibers to be handled cleanly with minimum waste, high reproducability, and high line speed.
The term "carbonaceous" used herein refers to carbonaceous materials which have a carbon content of at least 65% and which are prepared by the method described in the U.S. Pat. No. 4,837,076.